1. Field of the Invention
This invention relates to a method and apparatus for reforming hydrocarbon fuels to produce hydrogen in which extracted waste heat from internal combustion engines, furnace exhaust gases and the like at temperatures of about 1500° F. and lower and pressures ranging from a few inches of water column above atmospheric to about 10 bar is utilized to increase the temperature of a gaseous or liquid fuel, in a mixture with steam, to a level where the heated mixture will begin to react in the presence of a reforming catalyst and consume the extracted waste heat until the temperature of the catalyzed reactants becomes so low that the reaction virtually ceases. This invention may also be applied to other homogeneous or heterogeneous reactions where large amounts of reaction heats are either consumed or liberated. Without intending in any way to limit the scope of this invention, the invention as described herein is based upon the example of a catalyzed reaction with strong endothermic heat consumption.
2. Description of Related Art
Thermal processes often reject large amounts of heat. The percentage of rejected or waste heat is particularly large in processes in which chemical energy or fuel value is converted into mechanical energy. Exemplary of such processes are engines. Reciprocating internal combustion engines have thermal efficiencies in the range of about 25 to 40% depending upon design and age of the engine. Diesel engines typically have higher efficiencies than gasoline engines. A typical, modem diesel engine may have a mechanical efficiency of about 35%. Thus, depending upon the type of engine employed, up to about 75% of the fuel value consumed by these engines is converted into waste heat. Since the invention of these engines, efforts have been ongoing to increase their mechanical efficiency; and as fuel costs increase, these efforts become more urgent.
One approach for utilizing part of the waste heat generated by reciprocating internal combustion engines is thermochemical recuperation or TCR. In this process, a portion of the waste heat is recirculated into the engine. A method commonly employed in boilers and other heating devices, namely preheating of combustion air by exchanging heat in a heat exchanger, is not readily applicable to engines because the mass of preheated air or gas aspirated by the engine will be much smaller and will lead to performance derating. In addition, engine cooling will become more complex.
Reforming of fuels with steam is an established art. In this process, a gaseous or liquid fuel such as natural gas or methanol is reacted at high temperatures, greater than about 1500° F., to produce hydrogen, carbon monoxide, and carbon dioxide. The use of such high temperatures permits the use of thermal radiation both outside the reformer reactor tubes and, more importantly, inside the reactor tubes. However, when dealing with lower temperatures, such as are encountered with exhaust gases from engines and many furnace applications, heat transfer mechanisms change drastically from those at higher temperatures, especially those occurring in packed catalyst beds with strongly endothermic reactions.
The reforming reaction can follow different paths based on process conditions and will, accordingly, produce a variety of reaction end products. When producing large percentages of hydrogen and carbon monoxide, the reaction is endothermic; that is, the reaction consumes heat. In the reforming reaction, the consumed, sensible heat is converted into fuel with a higher heating value. Thus, comparatively low-cost waste heat is converted into a higher heating value fuel.
U.S. Pat. No. 6,855,272 B2 to Burlingame et al. teaches a syngas production process and reforming exchanger in which a first portion of hydrocarbon feed mixed with steam and oxidant is passed through an autothermal catalytic steam reforming zone to form a first reformed gas of reduced hydrocarbon content, a second portion of the hydrocarbon feed mixed with steam is passed through an endothermic catalytic steam reforming zone to form a second reformed gas of reduced hydrocarbon content, and the first and second portions of reformed gases are mixed, forming a gas mixture which is passed through a heat exchange zone for cooling the gas mixture and, thereby, providing heat to the endothermic catalytic steam reforming zone. The endothermic catalytic steam reforming zone and the heat exchange zone are respectively disposed tube side and shell side within a shell-and-tube reforming exchanger, which comprises a plurality of tubes packed with low pressure drop catalyst-bearing monolithic structures.
The reforming of fuels with steam is readily applicable to reciprocating, internal combustion engines, gas turbines, and furnaces. Utilization of the waste heat from an internal combustion engine for reforming of fuels is taught, for example, by U.S. Pat. No. 6,508,209 B1 to Collier, Jr. in which natural gas and/or propane is introduced into a reforming reactor for the purpose of converting or reforming a portion thereof to hydrogen and carbon monoxide, providing a gaseous mixture exiting the reactor comprising methane and/or propane, hydrogen, steam, nitrogen, carbon monoxide, and carbon dioxide. The gaseous mixture is mixed with air to provide a gaseous fuel mixture and air combination which is introduced into the internal combustion engine and combusted to produce an exhaust gas. A portion of the exhaust gas is recycled and introduced into the reforming reactor for the purpose of reforming a portion of the gaseous fuel to hydrogen and carbon monoxide. In accordance with one embodiment, the exhaust gas is used, without diluting the combustion charge, for preheating the fuel to be reformed, as well as the catalyst bed, for purposes of reforming the fuel.
However, despite its apparent attractiveness, this method has not found widespread use due to a number of technical problems. These problems must be resolved before this method can be effectively applied to reciprocating, internal combustion engines, gas turbines and furnaces. For one thing, the amount of heat that can be saved is very much a function of exhaust gas temperature. The higher the exhaust gas temperature is and the lower the temperature of the discharged, cooled exhaust gas is, the more waste heat that can be recovered. On the other hand, elevated temperatures are required to initiate the reforming reactions. These reforming reactions are strongly temperature dependent. When the hot mixture of fuel and steam contacts a suitable reforming catalyst, the reforming reactions will be initiated. However, they will extinguish themselves quickly if no heat is supplied to keep the reaction temperatures at a high enough level to maintain the reforming reactions.